Magnetic lens device for producing magnetic fields with an even number of four or more poles



y 1964 M. SCHEER ETAL MAGNETIC LENS DEVICE FOR PRODUCING MAGNETIC FIELDS WITH AN EVEN NUMBER OF FOUR OR MORE POLES Flled July 25, 1962 United States Patent 3,141,117 MAGNETIC LENS DEVICE FOR PRODUCING MAG- NETIC FIELDS WITH AN EVEN NUMBER OF FQUR 0R MORE POLES Max Scheer, Wurzburg, and Eberhard Keil, Offenbach am Main, Germany, assignors to Siemens-Schuckertwerke Aittiengesellschaft, Berlin-Siemensstadt, Germany, a

corporation of Germany Filed July 25, 1962, Ser. No. 217,508 Claims priority, application Germany Aug. 2, 1961 4 Claims. (Cl. 3172t 0) Our invention relates to lens devices for producing a magnetic field and particularly to such devices having an even number of four or more poles for action upon charged corpuscular radiation.

A known device for producing a magnetic four-pole field comprises individual poles located opposite each other in pairs and having a hyperbolic or rather an approximately hyperbolic surface, the poles being connected with one another by a closed yoke structure consisting preferably of a single iron body together with the poles. Each pole is provided with its own excitation winding. The winding sense of the respective windings is so chosen that poles of the same magnetic polarity are located opposite each other. Such four-pole lenses require a relatively large amount of iron as well as complicated winding shapes and must be produced with extreme precision. This renders themheavy in weight and expensive to manufacture.

It is an object of our invention to devise a magnetic lens for a four-pole field, and generally for a Zn-pole field (n being an integer greater than 1) that minimizes the above-mentioned disadvantages by affording, for given operating conditions, the provision of an iron structure of smaller weight and the use of field windings of relatively simple design and shape, without foregoing the possibility of producing virtually homogeneous magnetic fields.

According to the paper by Benaroya and Ramler in Nuclear Instruments and Methods 1961, No. 10, pages 113 to 120, it has become known to produce homogeneous fields by employing an iron core in form of a grooved stator as conventional for electric motors, the grooves containing field windings so arranged and traversed by electric current that distributed area currents occur in the cylinder-shaped hollow space of the iron body. The distributed area currents flow parallel to the cylinder axis and are distributed over the inner periphery of the cylinder space, the density of these currents being approximately proportional to the sine of the azimuth angle.

Employing such a grooved iron body, we provide according to our'invention a magnetic field-pole lens with an even number of four, six, eight or more poles, by maintaining the density of the area currents on the surface of the cylindrical hollow space substantially proportional to the sine of a value that constitutes the mathematical product of an integral number ni l times the azimuth angle a, wherein n denotes an integer larger than 1 and Zn is the number of field poles. Consequently, a Zn-pole field is obtained by an n-subdivision of the angle on with the aid of n-windings.

For example, according to the invention a four-pole lens is provided by distributing the sinusoidally distributed area-current density required for homogeneous fields, over one-half of the periphery and by doubling the winding. In other words, in contrast to conditions obtaining for a homogeneous field, it is necessary according to the invention to make the area-current density proportional to the sine of twice the azimuth angle a.

Under these conditions there obtains at the cylinder periphery a magnetic potential which is dependent upon the angle a in accordance with the equation:

wherein R denotes the radius of the hollow cylinder and c a field constant. For the case that all windings are traversed by the same current I and N denotes the number of turns (conductors) of the individual windings, it follows that By inserting Equation 2 into equation 1, one obtains OU N'I- sin 20:

for the xaxis by This accuracy of approximation to ideal condition achieved with a lens according to the invention, and consequently also the utilizable space in the middle of the hollow interior of the iron body, increases with an increase in the number of the grooves and/or with an increase in the number of individual winding conductors in the individual grooves. The relation between the abovementioned field constant c and the sum of the winding turn numbers EIN, in an octant is determined by 2 (6) c=EiN that is, c=? ZiN,

in which I denotes the current which flows through the series connectedwindings, the sum to be taken over an octant in the case of a four-pole lens.

The current intensity per centimeter periphery is given y (7) all Lenses whose field has 6, 8 or more poles can thus readily be provided by suitable windings in the grooves in accordance with the mathematical relations set forth in the foregoing. This will be further explained with reference to the embodiment of a magnetic lens according to the invention illustrated by way of example on the accompanying drawing.

The drawing shows only one quadrant of a four-pole field lens with a grooved iron body with a total of 72 grooves. The other quadrants are identical, considering only the alternating current direction, so that it is not necessary to show them on the drawing. The grooves are provided with field windings in a manner similar to that of conventional electric motors or other dynamo-electric machines. The individual conductors in the grooves all have the same cross section and are preferably connected in series with each other to be traversed by the same current, the series connection extending over one, two or all quadrants. The number of winding turns in each of (H R-cosnd the nine grooves N to N that occupy an octant differ from each other in accordance with the above-explained requirements. Permitting a maximum departure of 0.5% from the desired constant field gradient in an opening having the radius 0.8-R, the number of winding conductors in the sequence of grooves N to N is 23, 22, 21, 19, 16, 13, 10, 6 and 2 respectively. It will be seen that the distribution of the area currents over the cylinder surface is preferably effected in a stepwise, graduated manner. For other permissible departures from the ideal values, other winding numbers and a correspondingly different graduation will result.

The influence of integral-number conductors in the individual grooves upon the field characteristic in comparison with that of a smooth cylinder can be computed with sulficiently accurate approximation on the assumption that the lens has infinite axial length and that the windings consist of infinitely thin Wires located in the middle of the grooves on a circle having the radius R. The shares of the individual windings contributed to the field at a given locality can then be summed up in accordance with Biot-Savarts law. The above-given examples of correlated values for the winding-turn numbers in grooves N to N of the illustrated device are the results of such a computation. The magnetic field was computed, for example, for a square raster of points located Within a quadrant in the opening of the cylinder. The x-axis and y-axis were subdivided between and 0.9-R into steps of 0.1 -R. For a given value of R, one obtains in this example the value of the magnetic field by multiplying the above-mentioned numerical values or the number of winding turns with the term Field lenses according to the invention have the advantage of requiring comparatively little iron and little winding copper and may be given a magnetizable stator structure as conventional for electric motors. Such laminated stators further permit employing the lens for pulsating fields having four or more poles. The lenses according to the invention further do not require the use of complicated trapezoidal windings as is the case with the known lenses heretofore available for similar purposes.

It should be noted that the above-mentioned publication Nuclear Instruments and Methods is published by the North Holland Publishing Company.

It should also be noted that the illustrated magnetic lens is made of laminated sheets having the shape of a machine armature. The lens possesses an outer diameter of 214 mm. and an inner diameter of 150 mm., with a length of 300 mm. As stated, 72 radial grooves are uniforrnly distributed in the inner periphery, each extending along the length of the lens. This provides, in a four-pole lens, 18 grooves per sector, the grooves in each sector being designated N (at one edge of each sector) to N and to N (at the other edge of the sector).

Used for the windings is enamel insulatd copper wire having a 1 mm. diameter. The electrical current during continuous use may have a value of approximately amperes. Transient currents may then have a corre- 5 spondingly higher value.

We claim:

1. A magnetic lens comprising a hollow body having a cylindrical inner opening of circular cross section and having 221 magnetic field poles, n being an integer greater 10 than unity, and electric current conductors extending parallel to the cylinder axis and distributed along the cylinder periphery in accordance with a density of the area current on the cylinder surface proportional to the sine of n-x, wherein x denotes the azimuth angle, said conductors being wound in directions to produce opposing polarities in adjacent poles.

2. A magnetic lens comprising a magnetizable ringshaped body having a substantially cylindrical lens opening of circular cross section and having peripherally sequential magnet-field poles of alternately different polarity equally spaced from each other, the number of said poles being 211, with n being an integer greater than unity, electric conductors extending parallel to the cylinder axis and inductively linked in groups with said respective poles,

the number of conductors in sequential groups being graduated in accordance with proportionality of the areacurrent density to the sine of tthe value n-x, wherein x denotes the azimuth angle, said conductors being wound in directions to produce opposing polarities in adjacent poles.

3. In a magnetic lens according to claim 2, said ringshaped body being a hollow-cylindrical iron structure having grooves extending radially outwardly from said opening to form said poles intermediate said grooves, and

said conductors consisting of windings disposed in said grooves, said conductors being wound in directions to produce opposing polarities in adjacent poles.

4. In a magnetic lens according to claim 3, said conductors of said windings having all the same cross section 40 and the same current magnitude, and the number of conductors in respective sequential grooves being different in accordance with said proportionality, said conductors being wound in directions to produce opposing polarities in adjacent poles.

OTHER REFERENCES Benaroya et al.: Deflection Coil for an External Accelerator Beam, Nuclear Instruments and Methods, vol. 10, No. 2, February 1961, pages 113-120. Published by 5 North-Holland Pub. Co. of Amsterdam. 

1. A MAGNETIC LENS COMPRISING A HOLLOW BODY HAVING A CYLINDRICAL INNER OPENING OF CIRCULAR CROSS SECTION AND HAVING 2N MAGNETIC FIELD POLES, N BEING AN INTEGER GREATER THAN UNITY, AND ELECTRIC CURRENT CONDUCTORS EXTENDING PARALLEL TO THE CYLINDER AXIS AND DISTRIBUTED ALONG THE CYLINDER PERIPHERY IN ACCORDANCE WITH A DENSITY OF THE AREA CURRENT ON THE CYLINDER SURFACE PROPORTIONAL TO THE SINE OF N.X, WHEREIN X DENOTES THE AZIMUTH ANGLE, SAID CONDUCTORS BEING WOUND IN DIRECTIONS TO PRODUCE OPPOSING POLARITIES IN ADJACENT POLES. 